Scalable video coding schemes have been used for compressing video transmitted over computer networks with varying bandwidths, such as the Internet. One well known type of scalable video coding scheme is fine granular scalable (FGS) coding. The FGS video coding scheme has been adopted by the ISO MPEG-4 standard as the core video coding method for the MPEG-4 Streaming Video Profile.
As shown in FIG. 1, the FGS video coding scheme, hereinafter referred to as FGS, includes a prediction-based base layer 10 coded at a bitrate RBL and a single enhancement layer 11 coded using a fine-granular scalable (or embedded) coding scheme to a maximum bitrate of REL.
The FGS video coding scheme of FIG. 1 is very flexible because it does not use motion-compensation in the enhancement layer 11. Thus, each enhancement layer frame can be truncated depending upon the available bandwidth at transmission time. However, the lack of motion-compensation in the enhancement layer 11 decreases the image quality of the video.
FIG. 2 shows an improved FGS coding scheme that uses motion-compensation in the enhancement layer to improve the image quality of the video. This improved FGS coding scheme is hereinafter referred to as MC-FGS video coding, is described in U.S. patent application Ser. No. 09/887,756 entitled SINGLE-LOOP MOTION-COMPENSATION FINE GRANULAR SCALABILITY filed on Jun. 22, 2001 by the assignee herein, the entire disclosure of which is incorporated herein by reference.
The MC-FGS video coding scheme of FIG. 2 also includes a prediction-based base layer 20 coded at a bitrate Rb and a single enhancement layer 21 coded using a fine-granular scalable (or embedded) coding scheme to a maximum bitrate of Re. However, unlike the FGS coding scheme, where the P and B base layer frames are predicted from base layer I and P reference frames, the P and B frames of the base layer 20 of the MC-FGS coding scheme are predicted from motion-compensated “extended” or “enhanced” base layer I and P reference frames (hereinafter extended base layer reference frames) during base layer coding. Each motion-compensated extended base layer reference frame comprises data from a standard base layer reference frame, data from at least a portion of an associated enhancement layer reference frame (one or more bitplanes or fractional bit-planes of the associated enhancement layer reference frame can be used), and motion estimation data.
FIG. 3 graphically compares the performances of the FGS and MC-FGS video coding schemes. As can be seen, the MC-FGS video coding scheme has higher peak signal-to-noise ratio (PSNR) values, (PSNR is a measure of quality for each frame) than the FGS video coding scheme at higher bitrates, i.e., bitrates above about 280 kbit/s in FIG. 3. However, at the lower bitrates, i.e., below about 280 kbit/s in FIG. 3, the FGS coding scheme outperforms the MC-FGS coding scheme because of introduced prediction drift in MC-FGS coding scheme. This drift is caused by the use of enhancement layer frame data in the motion-compensation prediction of the base layer P and B frames. Since enhancement layer frame data is only available at the decoder at bitrates greater than RBL, prediction drift will occur in the MC-FGS coding scheme at the lower bit-rates where this enhancement layer data is not available.
Accordingly, there is a need for an MC-FGS video coding scheme that has reduced prediction drift at low bitrates.